Here's why, according to the Associated Press (hint: it has a lot to do with that continued aid):

The administration has been forced into difficult contortions to justify not declaring a coup d'etat, which would prompt the automatic suspension of American assistance programs under U.S. law. Washington fears that halting such funding could imperil programs that help to secure Israel’s border and fight weapons smuggling into the Hamas-controlled Gaza Strip, among other things seen as critical to U.S. national security.

The non-decision leaves a door open for the administration to call the 2013 overthrow of the Egyptian government a coup in the future, but that would probably happen only if the U.S. loosened a restriction requiring the government to cut off all foreign aid to a country in the event of a coup. There are already a few members of Congress interested in pursuing those changes, including Republican senators Jim Inhofe and Bob Corker.

The administration's refusal to call the events a coup puts them at odds with the AP's go-ahead earlier this month for their journalists (and the many, many journalists outside of the AP who follow their style guide) to use the word "coup" to describe the governmental overthrow in Egypt, so long as reporters specify that the military takeover occurred in the midst of a popular uprising against the government. The AP's decision was prompted by the military's crackdown on Morsi supporters in the wake of the takeover, and the dissolution of its elected parliament.